Embodiments described herein relate to wireless monitoring. Wireless monitoring is useful in many situations. For example, wireless monitoring may be used in health care to monitor physiological values for a patient, and to call attention to changes in physiological values that may indicate a condition requiring attention. By using wireless monitoring instead of conventional wired instrumentation, patients are free to move around within, for example, a home or physician's office or hospital, rather than being tethered within a small area by electrical cables.
Wireless monitoring may also be used, for example, to monitor the location of a person who might be endangered by straying, such as a child or an elderly person whose memory is impaired. Wireless monitoring may also be used, for example, to monitor conditions or processes or equipment in settings where wired monitoring is inconvenient or difficult. In a chemical plant, for example, it may be convenient to attach wireless monitors to pipes or vessels or other equipment rather than using wired monitors that require electrical cables or cords.
FIG. 1 is a block diagram of a typical wireless monitoring system. A remote unit 30 may be affixed to a patient for use in a health care setting. Data 41 indicating one or more physiological values are collected by remote unit 30 and transmitted via a wireless medium 14 to a base station 20. The physiological values monitored may include, for example, values for blood pressure or oxygen saturation or heart electrical activity (electrocardiogram). The base station 20 may also send data to the remote unit 30, such as acknowledgement of successful reception of data, request for battery status, and so forth. The data 41 received from the remote unit 30 are available at the base station 20 for observation or storage or for retransmission over a connection 121, as appropriate. For example, data 41 may be transmitted to a data processing device 122. Remote unit 30 typically is powered by a storage battery, while base station 20 typically is powered from the alternating current (AC) mains. Wireless communication may be acoustic, optical, or radio, radio normally being preferred as it does not typically require an unobstructed line of sight between sender and receiver.
The utility of a wireless monitoring system is dependent on several attributes of the system, including:    (1) Size (dimensions) of the remote unit 30: smaller is usually better;    (2) Weight of the remote unit 30: lighter is usually better;    (3) Range over which the remote unit 30 may roam while maintaining wireless contact with the base station 20: greater range is better; and    (4) Battery lifetime (operational time of the remote unit 30 before the battery discharges): longer is better.
These attributes are interdependent, and optimizing one attribute often has a negative effect on another. For example, a simple way to increase range is to increase the transmission strength of the wireless signal. Transmitting a stronger signal, however, requires more electrical current per unit time, which can only be achieved by decreasing battery lifetime and/or by increasing battery capacity (thereby increasing the size and weight of remote unit 30). Conversely, decreasing the size and weight of the remote unit 30 by using a smaller battery will normally decrease available power, and thus negatively impact the range and/or battery life of the remote unit 30.
The trade-off between battery size and weight versus wireless range and battery lifetime is particularly important because the battery typically is the largest and heaviest component of a contemporary remote unit 30. Thus, a change in battery size and weight will normally have an important effect on the size and weight of remote unit 30. Minimizing the power requirements of remote unit 30 can help to reduce the size and weight of remote unit 30. Small size and weight may be especially advantageous in certain monitoring systems, such as a health care monitoring system where remote unit 30 is mounted upon a patient.
In many instances, it may be desirable for the remote unit 30 to communicate wirelessly with a plurality of base stations 20. For example, it may be useful to transmit data 41 to several locations simultaneously. In a health care monitoring system, for example, it may be advantageous to transmit physiological values for a patient to several nursing stations, each having a base station 20. Also, the availability of multiple base stations 20 can increase the area over which a patient may roam while still maintaining contact with at least one base station 20. FIG. 2 is a block diagram of a wireless monitoring system that includes a remote unit 30 and four base stations 20. Each base station 20 communicates with remote unit 30 via a wireless medium 14.
FIG. 3 depicts the zone of coverage 123 for a single base station 20. The actual size and shape of the zone of coverage 123 depends upon many factors, including the power of the transmission, the sensitivity of the receiver, the type of antenna, and interfering surfaces and objects. FIG. 4 depicts the zone of coverage 123 afforded by a plurality of base stations 20. The zone of coverage 123 provided by multiple base stations 20, which is depicted in FIG. 4, is much larger than the zone of coverage 123 for a single base station 20, which is depicted in FIG. 3.
Increasing the number of base stations 20 in communication with a remote unit 30 poses several complications, however. One complication is the potential need to ensure that all base stations 20 receive data 41 transmitted by the remote unit 30, even if not all base stations 20 are currently in contact with the remote unit 30. Another complication is the need to coordinate transmissions from individual base stations 20 to the remote unit 30. If several base stations 20 attempt to communicate with the remote unit 30 at the same time, then the signals may “collide”, rendering all communications unreadable by the remote unit 30.
Coordination of tranmissions from individual base stations 20 may be achieved using any of various techniques, such as randomizing the transmission times for individual base stations 20 so that the transmissions times are unlikely to overlap, or by assigning unique transmission time slots to each base station 20. These techniques work well in systems that have plenty of available power, such as systems that are powered from AC mains power, yet have a subtle but important drawback in battery-powered devices, as is explained below.
In order for a message to be transmitted successfully, the transmitter must transmit and the receiver must receive. It is often not appreciated that in low-power systems a transceiver's receiver circuitry typically requires as much or more power to operate as does its transmission circuitry. Power used for message reception becomes particularly important in the case of a remote unit 30 that communicates with plural base stations 20. Remote unit 30 must receive various messages from the plural base stations 20, such as messages that acknowledge successful transmission of data 41, or messages that includes commands, or messages that request information about battery status. Typically, remote unit 30 must power its receiver circuitry to receive these messages from each of plural base stations 20.
Minimizing the power consumption of a device's transmission circuitry can be accomplished by powering the transmitter only when the device needs to send data. Minimizing the power consumption of a device's receiver circuitry is less straightforward: the device must predict the times when another device might be transmitting to it, and power the receiver circuitry during those times only.
Modern communication systems typically use data bursting technology. Rather than sending a continuous stream of data, these systems send bursts of data (known as packets), either periodically or as needed. In practice, low-power transceivers use algorithms to coordinate sending and receiving times of each transmission (burst). Time slots are agreed upon for transmission and reception to take place. FIG. 5 illustrates a hypothetical time course of transmission (T), reception (R), and power usage (P) for a low-power device such as a remote unit 30 that communicates with a single base station 20. The transmission circuitry is turned on during a first time slot and the receiver circuitry is turned on during a second time slot. Power is used during both the first time slot and the second time slot.
When a remote unit 30 needs to receive transmissions from a plurality of base stations 20, it must turn on its receiver circuitry for a long enough time to receive from each of the base stations 20 without collisions. This, however, significantly increases power consumption. FIG. 6 illustrates a hypothetical time course of transmission (T), reception (R), and power usage (P) for a low-power device such as a remote unit 30 that communicates with four base stations 20. The transmission circuitry is turned on during a first time slot and the receiver circuitry is turned on during four subsequent time slots. Power is used during five time slots, resulting in much greater power consumption than in the example of FIG. 5.
In wireless monitoring systems such as the system illustrated in FIG. 1, typically the remote unit 30 is a low-power or ultra-low-power device while considerable power is available to the base station 20. The remote unit 30 typically expends at least half (often up to 90%) of its power budget in data communication (transmission and reception). In addition to communicating with the remote unit 30, the base station 20 typically communicates with other entities using a network connection or other communication means.
Communication between the base station 20 and another entity may entail, for example, transferring data 41 to a second base station 20 or to another device that is part of a network of devices that includes base station 20. Base station 20, or another device in the network that includes base station 20, may also transfer data 41 to one or more distant entities, where distant entities are understood to be entities outside of the network. For example, data 41 may be transferred via a telephone or internet connection to a distant device such as the data processing device 122 depicted in FIG. 1. An alert about a condition that may require attention is another type of message that may be communicated to a distant entity. For example, an alert may be communicated using an audible signal, such as that produced by a bell or buzzer, in order to notify a human responder (a distant entity) to investigate the condition. An alert may also be communicated, for example, via a telephone or internet connection. Communication with distant entities typically is performed by devices such as a base station 20, and not by remote unit 30 itself, because remote unit 30 is a low-power device that may not be able to communicate with distant entities. While the term “base station” is used herein, it is understood that other terms such as “Access Point” may be used to describe a device that performs the function of communicating with distant entities; the term “Access Point” is used in the 802.11 communication protocol.
For a low-power remote unit 30 where it is desirable to minimize power consumption, the base station 20 should appear to be dedicated to the remote unit 30, when viewed from the perspective of the remote unit 30. This apparent dedication of the base station 20 prevents the remote unit 30 from wasting power due to the base station 20 not being ready at a given time. While the base station 20 may in fact communicate with other entities, as described previously, the base station 20 should appear to be dedicated to the remote unit 30.
In the simplest case, there is a single remote unit 30 at a fixed location and a single base station 20 at a fixed location; in other words, the system is a 2-element point-to-point network. Such as system requires either a dedicated communications channel or some initial handshaking to set up the connection parameters, but after that it can be quite predictable and reliable. Real-life examples are older cordless phones (that use a dedicated channel) and wireless devices that use handshaking to set up parameters, such as wireless devices that use the BLUETOOTH communication protocol.
In medical monitoring applications, it is common for the remote unit 30 to be mobile; for example, it may be attached to a human being. Mobility of the remote unit 30 raises several issues. One issue is that the remote unit 30 may come into and go out of range of an individual base station 20. Another issue is “handing over” the connection across multiple base stations 20 to avoid dropping the connection. In a system that includes a network with multiple base stations 20, it is extremely likely that individual base stations 20 will become unreachable and then reachable again, and will not come on-line at the same time. This set of issues related to mobility of the remote unit 30 and plurality of the base stations 20 requires a sophisticated algorithm to adapt the network to the changing conditions.
Dealing with the complexity described above can become very burdensome for an ultra-low-power device such as a remote unit 30, especially if the device needs to reliably transmit data at the same time. It may be desirable, therefore, to shift this complexity away from the remote unit 30. In a system with multiple base stations 20, each of which needs to receive data from the remote unit 30, one approach to avoiding complexity is to have the remote unit 30 transmit the same data to all base stations 20, and to have each base station 20 individually acknowledge receipt of the data. While this approach avoids some complexity, it increases the power consumption by the remote unit 30, as illustrated in FIG. 6.
There is a need for a communication system that enables a wireless remote unit 30 to communicate with multiple base stations 20 while consuming minimal power, as in the hypothetical time course of transmission (T), reception (R), and power usage (P) that is illustrated in FIG. 7.